This invention relates generally to thermal recording apparatus, and more particularly, to thermal recording apparatus in which a line of thermal resistive elements are selectively driven according to recording signals.
Thermal recording techniques have spread rapidly because of easy maintenance and clean recording. In such techniques, figures are usually recorded on a thermal sensitive paper by selectively actuating a line of recording elements (i.e., thermal resistive elements) in accordance with incoming recording signals.
Generally, in prior art thermal recording apparatus, a matrix circuit is used to decrease the number of drive elements and lead lines. A recently developed recording apparatus drives all the thermal resistive elements at the same time according to the recording signals without a matrix circuit; this apparatus results in high recording speed. In thermal recording apparatus using matrix circuits, each group of thermal resistive elements is driven in turn so all the thermal resistive elements have enough off time. On the other hand, in the recording apparatus in which all the thermal resistive elements are driven at the same time, there are occasions where the thermal resistive elements do not have enough off time because some elements may be continually driven. When these elements are continually driven, they overheat, which causes uneven recording density. Also, these elements sometimes break up due to overheating.
One method for preventing overheating is to decrease the supply of energy to these elements when these elements are continually driven. Such a method using monostable multivibrators is disclosed in Japanese patent publication No. 54-659. Namely, recording signals are supplied to drive circuits through monostable multivibrators triggered by recording signal pulses. The turn on time of the elements is equal to the time of the quasi-stable state of the monostable multivibrator. When the time between recording signal pulses (i.e., time between trigger pulses of the monostable multivibrator) is shorter than the intrinsic time of the quasi-stable state, the time of the quasi-stable state shortens and overheating is avoided.
The latter method has the disadvantage that it is difficult to optionally determine the ratio of turn on time Ta in case of turning on independently to turn on time Tb in case of turning on continually. Accordingly, it is difficult to avoid both overheating and unevenness of recording density.